Canton of Fribourg, Bridges and Pathways Service, Lakes and Rivers Section
2011 - 2013
Within the scope of the global project to map the natural dangers linked to rising water levels for the Fribourg Plateau region, BG is carrying out these studies for batch no.4 - Petite Glâne - Lac de Neuchâtel, no. 5 Sonnaz - Sarine - Ville de Fribourg and no.7 Lac de Morat. The entire study area therefore covers the Petite Glâne drainage basin, the tributaries of the river Broye and Neuchâtel lake, the downstream sections of the drainage basins of the river Sionge and Gruyère lake, the drainage basins of the Sonnaz and the La Crausaz rivers in the north-west of Fribourg, and the Bibera in the north-east with its tributaries and the Morat lake.
The map for batch no.5 and batch no.7 indicating rising water level dangers was initially drawn up by BG, using a large scale 2D hydraulic model for the whole study area. Cross-referencing this map (showing the areas potentially at risk of flooding) with the region’s existing issues made it possible to identify the plots which would benefit from being mapped during the next phase. This resulted in almost 90 km of rivers and 25 km of riverbanks across areas falling within more than 60 municipalities being covered by these danger maps.
The study of the dangers includes a first phase of collecting current data, site surveys, hydrological analysis (calculation of the flow rates) and analysis of any historical events that can establish danger scenarios to be taken into consideration. These are then studied in detail using different calculations and models in order to establish mapping of the actual dangers, in compliance with federal recommendations. Weak spots, in other words areas with the potential to be affected, are identified on this basis. A summary study of the associated economic risk enables the risk to be quantified.
Phase 1: Basic data
- Information collection
- Registry of historical events
- Land surveys
- Creation of a map indicating rising water level dangers
Phase 2: Study of phenomena
- Identification of potential phenomena
- Hydrological analysis
- Establishing the scenarios
Phase 3: Study of the dangers
- Detailed hydraulic analysis and establishing models
- Mapping of the intensities and dangers